1. Field of the Invention
The present invention relates, to a method of burning in a semiconductor device used in a microwave region, particularly in a region at a frequency of not less than 1 GHz.
2. Description of the Related Art
While compound semiconductors such as GaAs are used for transistors used in microwave regions (several hundreds of MHz to 100 GHz), particularly in a high-frequency region at a frequency of not less than 1 GHz, such compound semiconductors have various energy levels on the surface and inside. Some of these levels experience chronic changes such as diminishing concentrations due to electric stress given to the transistor, and, therefore, a burn-in process is carried out wherein electric stress is given to the transistor to cause changes beforehand in order to ensure reliability of the transistor.
FIG. 14 is a schematic diagram of a burn-in apparatus of the prior art. In the drawing, reference numeral 1 denotes a transistor, 2 denotes a package, and 3 denotes a matching circuit at the operating frequency. The semiconductor device being burned in is an internally matched FET having a matching circuit incorporated in the package.
Burn-in is carried out, with a predetermined drain voltage being applied to a transistor 1, by amplifying a frequency signal (for example 20 GHz) which is output from a signal source 5 by means of an amplifier 7. The signal is supplied to a gate of the transistor 1 while maintaining this state for a predetermined period of time and monitoring the power of the input and output microwave signals by means of a power monitor 6.
During such a burn-in operation, unless frequency matching is accurately done, a reflected wave is generated due to the parasitic inductance and capacitance of jigs and wirings, thus making it impossible to feed high-frequency signals to the transistor, and consequently it becomes difficult to carry out such matching in a high-frequency region not lower than 1 GHz. Also it becomes necessary to fabricate wiring of the matching circuit with a high dimensional accuracy in order to match the impedance. This results in higher cost of the matching circuit itself and higher total cost including peripheral devices such as the amplifier.
Meanwhile the present inventors have found that, while burn-in is carried out with a high-frequency electrical stress applied, the effect of burn-in may also be achieved by using a frequency lower than the operating frequency, that the impurities do not respond to frequencies lower than a certain value and are mobilized, thus making it impossible to achieve the same burn-in effect as that carried-out at the operating frequency.
That is, because impurities have different response frequencies (reciprocal of response speed), the impurities cannot follow the changes in the electrical field and move accordingly in the case of burn-in with an alternating electric field of a frequency higher than the response frequency of the impurity level. When the burn-in frequency is set lower than the response frequency, however, the impurities move in a behavior different from that observed during burn-in at the operating frequency where the impurities do not move, thus the desired burn-in effect cannot be achieved.
According to the inventor""s knowledge, the frequency at which the impurities at the impurity level begin to move is 100 MHz or lower for semiconductors such as GaAs, and the physical behavior of impurities remains the same whether the frequency is 1 GHz or 100 GHz, if the frequency is higher than 100 MHz.
At a frequency higher than 100 MHz, thermal effects can be prevented because this frequency is sufficiently higher than the transient thermal response frequency of semiconductor which is several megahertz.
Based on the finding described above, it is assumed that a burn-in effect similar to that achieved at the actual operating frequency, for example, 18 GHz or 40 GHz, can be obtained by burning in-at a frequency higher than 100 MHz.
Japanese Patent Kokai Publication No. 60-33066 discloses a method of burning in by applying a low-frequency signal (10 MHz or lower) to a gate electrode of a high-frequency transistor. However, because impurities at the impurity level begin to move at a low frequency, such as 10 MHz or lower, it will not be possible to achieve the same burn-in effect as that obtained with the operating frequency (100 MHz to 20 GHz).
An object of the present invention is to provide a method of producing a semiconductor device having the same burn-in effect as is achieved by burning in at the operating frequency, through burn-in at a frequency lower than the operating frequency of a high-frequency transistor.
The present invention provides a method of burning in semiconductor transistors by supplying a signal of burn-in frequency to the semiconductor transistors used in a microwave region, wherein the burn-in frequency is set lower than the operating frequency of the semiconductor transistor device and is higher than the response frequency of the impurities included at the impurity level.
By employing such a method as described above, the same burn-in effect as is achieved by burning in at the operating frequency can be achieved, even when burn-in is carried out at a frequency, lower than the actual operating frequency (microwave region from several hundreds of mega-hertz to 100 GHz).
The burn-in frequency is preferably higher than the transient thermal response frequency of the semiconductor transistor.
This is because, when burned in at a frequency higher than the transient thermal response frequency, heat due to transient thermal response is not generated during burn-in process and therefore burn-in condition can be prevented from changing.
The operating frequency is preferably higher than 1 GHz.
This is because it makes it possible to burn in a semiconductor transistor of a high operating frequency at a lower burn-in frequency.
The burn-in frequency is preferably selected from the range from 10 MHz to 1 GHz.
The burn-in frequency is more preferably selected from the range from 100 MHz to 1 GHz.
The semiconductor transistor device may also consist of an input matching circuit for the operating frequency, the semiconductor transistor and an output matching circuit for the operating frequency.
The burn-in operation may also be done by connecting an input/output matching circuit for the burn-in frequency to the semiconductor transistor device.
The semiconductor transistor may also be put in class A operation by using a resistor R for the load of the output matching circuit.
This is because such a method enables it to burn in a semiconductor transistor used in class A operation.
The semiconductor transistor may also be put in class C operation by using a resistor Rand an LC parallel circuit which is connected in parallel with the resistor R and resonates at the burn-in frequency for the load of the output matching circuit.
This is because such a method enables it to burn in a semiconductor transistor used in class C operation.
The semiconductor transistor may also be put in class F operation by losing a resistor R, a first LC parallel circuit which is connected in parallel with the resistor R and resonates at the burn-in frequency and a second LC parallel circuit which is connected in series between the resistor R and the first LC parallel circuit and the transistor output and resonates at a frequency three times the burn-in frequency, for the load of the output matching circuit.
This is because such a method enables it to burn in a semiconductor transistor used in class F operation.
The present invention also provides a burn-in apparatus for burning in the semiconductor transistor used in microwave region by supplying a signal of burn-in frequency, comprising a burn-in frequency signal source, a semiconductor transistor device with an input connected to the signal source, and a load connected to an output of the semiconductor transistor device, wherein the burn-in frequency is lower than the operating frequency of the semiconductor transistor device and is higher than the response frequency of impurities included at the impurity level of the semiconductor transistor.
Use of such a burn-in apparatus makes it possible to easily achieve matching at the burn-in frequency.
The semiconductor transistor device preferably incorporates an input matching circuit for the operating frequency, the semiconductor transistor and an output matching circuit for the operating frequency.
The load may also comprise a resistor R to have the semiconductor transistor function in class A operation.
This is because harmonics will not be generated in class A operation, and use of the resistor R makes burn-in possible.
The load may also comprise a resistor R and a LC parallel circuit which is connected to the resistor R in parallel and resonates at the burn-in frequency to have the semiconductor transistor function in class C operation.
This is because generation of harmonics must be taken into consideration in the case of class C operation, and use of the load of such a configuration makes it possible to load the semiconductor transistor.
The load may also comprise a resistor R a first LC parallel circuit which is connected to the resistor R in parallel and resonates at the burn-in frequency and a second LC parallel circuit which is connected in series-between the resistor R and the first LC parallel circuit and the transistor output and resonates at a frequency three times the burn-in frequency, to have the semiconductor transistor function in class F operation.
This is because generation of harmonics must be taken into consideration also in the case of class F operation, and use of the load of such a configuration makes it possible to load the semiconductor transistor.
The semiconductor transistor may also be mounted in a burn-in package with the package incorporating an attenuator circuit mounted therein for attenuating frequencies higher than the burn-in frequency.
This is because use of such a low-cost low-frequency package makes it possible to decrease the package cost and mounting the attenuator circuit in the package makes it possible to prevent oscillation during burn-in process.
The attenuator circuit preferably comprises a CR series circuit.
This is because such a configuration makes it possible to attenuate oscillation during burn-in process most simply and at a low cost.
The load may also be configured to substantially match the actual operation parameters of the semiconductor transistor which are determined by measuring drain voltage and drain current of output signal of the semiconductor transistor.
The semiconductor transistor device may also have an input matching circuit provided at the input side thereof for resistance matching.
The present invention also provides a semiconductor transistor which is burned in.
Burning in by such methods makes it possible to improve the reliability of semiconductor transistors operating at high frequencies.
According to the present invention, as described above, it is made possible to carry out burn-in with a lower frequency while preventing undesirable oscillation, by using a low-frequency package of lower cost with C and R added thereto for attenuating high-frequency components added therein.
Although the high-impedance probe is used instead of a power sensor in this embodiment, electric stress to the transistor may also be determined through calculation of voltage and current because the voltage can be directly measured at a low frequency such as 800 MHz.
While the first to fourth embodiments relate to the burn-in process of FET of operating frequency 18 GHz, burn-in at 800 MHz can be done with the same method for FETs of different operating frequencies such as 40 GHz and 10 GHz.
As will be clear from the foregoing discussion, the burn-in method of the present invention makes it possible to achieve the same burn-in effect as that achieved by burning in at the operating frequency, through burn-in at a burn-in frequency lower than the actual operating frequency in microwave region (several hundreds of mega-hertz to 100 GHz).
This makes it possible to achieve easy matching at the burn-in frequency and reduce the production processes.
It is also made possible to prevent the burn-in conditions from changing, because heat generation due to transient thermal response does not occur during burn-in as the burn-in process is carried out at a frequency higher than the transient thermal response frequency.
The frequency of the high-frequency signal is preferably selected from the range from 10 MHz to 1 GHz, and more preferably selected from the range from 100 MHz to 1 GHz.
By using the burn-in apparatus of the present invention, it is made possible to carry out burn-in with a low-cost apparatus and reduce the production cost.
It is made possible to apply sufficient load to a semiconductor transistor while taking the generation of harmonics into consideration, by using a resistor R for the load when the semiconductor transistor is used in class A operation, by using a resistor R and an LC parallel circuit which is connected in parallel with the resistor R and, resonates at the burn-in frequency for the load when the semiconductor transistor is used in class C operation, and by using a resistor R, a first LC parallel circuit which is connected in parallel with the resistor R and resonates at the burn-in frequency and a second LC parallel circuit which is connected in series between the resistor R and the first LC parallel circuit and the transistor output and resonates at a frequency three times the burn-in frequency when the semiconductor transistor is used in class F operation.
By incorporating the semiconductor transistor in a package and providing an attenuation circuit for attenuating components of frequencies higher than the burn-in frequency in the package, it is made possible to reduce the package cost and prevent oscillation from occurring during burn-in.
Also high reliability can be achieved for semiconductor transistors burned in by the method of the present invention.